All of them fit perfectly. We are just more used to see the "classics": Luffy, Zoro, Sanji, Nami and Usopp, because they joined a long time ago and when the crew had few numbers.
Later came Chopper and Robin, who we are now used, but both were really weird to see when they joined. At the time, I felt both would not fit the crew.
Franky and Brook have joined later, and since them we have little time to get used to them. To make matters worse, the crew is pretty big right now, so their screen time is reduced. Just try to reread scenes where they shine, and you'll see they fit perfectly.
For Franky:
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Franky, Sogeking and Sanji on the sea train;
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Strawhats (except Nami, Usopp and Chopper) entering Thriller Bark and being captured one by one. Keep reading until only Franky and Robin are left and fight Taleran.
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Later, Franky and Robin save Usopp & Chopper and they return to the ship.
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Fight against Odz, specially the first few chapters before Franky's KO'ed. Special attention to the Big Emperor robot scene.
Franky has great sinergy with Robin, Chopper and Usopp, and his personality has stark contrast to Zoro's.
For Brook:
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The fight against the flying fish riders. Special attention to his interactions with Zoro and Hachi;
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Sabaody Archipelago scenes, with him, Luffy, Chopper, Cammie and Pappag.
Brook seems to have great sinergy with Zoro, Chopper and Luffy. However, his interactions with the crew have been really limited so far.